Cathode ray tube television and like apparatus



Nov. 19, "1940. E. L. c. WHITE ErAL" 2 ,4 6

CATHODE RAY TUBE TELEVISION AND LIKE APPARATUS Filed Oct. 8 1938 3 Sheets-Sheet l Aanp lifief g I x Ouipwi fi'ame .Sweep Generator wvgwms ERIC-LAWRENCE (ASL/N6 WHITE CHARLES LESLIE FZUDELL ORNEY Nov. 19, I 5 c, w n- ETAL 2,222,426

CAT HODE RAY TUBE TELEVISION AND LIKE APPARATUS Filed Oct. 8, 1938 I s Sheets-Sheet 2 Relay R 14.1. SUPPLY AAAAAAA 'AAAAAAA v n y IN VENTORS ERIC LA WRENCE CASL/NG WHITE CHARLES L ESL/E FA UDELL ATTORNEY NOV. 1 9, 1940. E c wHlTE ETAL 2,222,426

CATHODE RAY TUBE TELEVISION AND LIKE APPARATUS Filed Oct. 8, 1938 3 Sheets-Sheet 3 RZ0.y

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E Z? Lme Sweep Frame Sweep- Generafar nerator v /NVENTOR$ CHARLES LESI. FAUDELL ATTORNEY Patented Nov. 19, 1940 UNITED STATES CATHODEJRAY TUBE TELEVISION AND LIKE I APPARATUS Eric Lawrence Gasling White, Hillingdon, and Charles Leslie Faudell, Stoke Poges, England, assignors to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application October s, 1938, Serial No. 233,926

in Great Britain October 11, 1937 6 Claims. (01. 250-27) 7 l each of said signals is appliedto a rectifier "and The present invention relates to television and like systems and has for its object to provide improved means to protect the fluorescent or mosaic screen of a cathode ray tube or like piece Fr of apparatus from damage by an intense beam of cathode rays which becomes stationary or which scans only a single line due to non-operation of both or one of the scanning circuits,

The beam of cathode rays is usually accelerated by a potential of several thousand volts and thus possesses considerable kinetic energy which is normally absorbed by the whole screen area. If failure of the scanning apparatus causes the 31 beam. to be concentrated on a stationary spot of the screen, damage to that part of the screen is likely to result, and even if the beam moves back and forth over a single line damage may occur. This is particularly so when very high accelerat ing potentials are used, for example, in the case of so-called projection receivers in which a very bright picture is required to be formed on the cathode ray tube screen and to be projected by an optical system on to a much larger external 25 viewing screen. Cathode ray tubes of the type known as the iconos'c'ope tube and used for transmitting pictures are also supplied with very high operating potentials and in this case the photo-electric mosaic screen is liable to damage 30 in the event of failure of the scanning apparatus.

It is an object of the present invention to provide means whereby damage to the screen as a result of failure of the'scanning apparatus is substantially avoided. 35 According to a feature of the invention, television or like apparatus comprises a cathode .ray tube and a pair of deflecting circuits for producing beam deflecting signals, wherein potentials derived from said deflecting signals are '40 applied to at least two control electrodes of a thermionic valve so as normally to maintain the amplitude of one or more of said deflecting the rectified potentials are applied through suitable impedances to the control electrode of a.

thermionic valve so as normally to maintain said valve in a predetermined condition, and wherein means are provided for preventing the rectified potentials from'exceeding a predetermined Value,

the arrangement being such that when the ariiplitude of either of said deflecting signals falls below a predetermined value the condition of said valve is changed so that the latter functions to render the beam substantially innocuous to the screen of the tube.

According to a further feature of the invention, television apparatus is provided comprising a cathode ray tube,'wherein frame and line deuous to the screen of said tube, wherein the timeconstants of the rectifier circuit, or circuits'is'br are so chosen that'in the event of the amplitude of the line deflecting signals falling below said predetermined value the beam is rendered substantially innocuous to the screen'within sub-- stantially a few lines from the instant thatsaid event occurs.

According to a still further ieature of the, in-

vention, television apparatus is provided comprising a cathode ray tube,wherein frame and line deflecting signals are arranged to be applied to the electron beam, and wherein the line deflecting signals'are obtained from a so-called resonant return circuit, (similar to that described in the Specification of British Patent No. 400,976),

which includes-a rectifier which isrendered conductive at predetermined instants, and, wherein the potentials from said rectifier are applied together with potentials obtained from a rectifier associated with the frame deflecting signals to a thermionic valve to maintain it in" a .predeter mined condition and is so arranged that should either of said rectifier potentials fall below. a

predetermined value'the condition of said valve changes so that the latter functions to render the beam substantially, innocuous to thescreen ofsaidtube. r I Apparatus according to the invention may be so arranged that when the condition of the previously mentioned thermionic valve is changed a relay is brought into operation in order to interrupt the power supply which supplies operating potentials to one or more accelerating electrodes of the cathode ray tube. Alternatively or in addition, means may be provided whereby when the condition of said thermionic valve changes the accelerating electrode may be effectively shortcircuited to the cathode of the cathode ray tube.

Other features of the invention will be apparent from the accompanying description and the appended claims.

Several embodiments of the invention will be described by way of example, reference being made to the accompanying drawings.

Embodiments of the present invention are illustrated in the figures of the accompanying drawings of which Figure 1 shows protective arrangements applied to a cathode ray tube television receiver,

Figure 2 shows protective arrangements applied to a television transmitting tube of the iconoscope type,

Figures 3 and 4 show protective arrangements for the screen of a cathode ray tube comprising a hexode type of valve, and

Figure 5 of the accompanying drawings shows protective arrangements for the screen of a cathode ray tube comprising a double diode triode type of valve.

Referring to Figure 1 of the drawings the cathode ray tube CR is provided with magnetic scanning coils Sr. and SF which carry respectively currents of line scanning andframe scanning frequencies. The accelerating electrodes A1 and A2 of the cathode ray tube are supplied with high tension voltage by the rectifier units R1 and R2 which are connected to the supply mains AC via a relay contact 3 controlled by the relay coil 4. The voltages normally applied to A1 and A2 may be, for example 2,000 volts and 20,000 volts respectively. The valve V1 is an amplifier to the control grid 5 of which are applied line frequency scanning oscillations generated by a separate sawtooth oscillator not shown in the drawings. The output of valve V1 is applied via the transformer TL to the line scanning coil SL. The valve V3 operates similarly to amplify frame scanning oscillations and to deliver deflecting current to the frame scanning coil SF.

The contact 3 is normally held closed by direct current in the coil 4 which also flows through a control valve V4. The valve V4 is arranged to pass a direct current dependent upon the amplitude of the scanning currents delivered by the scanning output valves V1 and V3, and if this direct current falls below a prearranged value the contact 3 opens thus breaking the main supply to the high tension rectifier units. An important feature of the invention previously mentioned is the provision of means which simultaneously discharges one or more of the high tension units, and this is accomplished in the present arrangement by a valve relay V5 which becomes conducting when its grid potential, derived from the anode of V4, reaches a certain value. As the current through V; decreases, its anode potential rises nearer to that of the high tension line 6, feeding the scanning generators and to which the cathode of V5 is connected, and consequently the grid 1 of V5 becomes less negative with respect to its. cathode. The anode 8 of V5 is connected to the cathode ray tube high tension line 9 which is, therefore, discharged through V5 to the comparatively low voltage line 6 (usually 300 volts or less).

The current passed by the control valve V4 is regulated by control potentials derived from the scanning output valves V1 and V3. An additional winding ii! on the frame scanning transformer TF supplies a rectifier circuit comprising a rectifier H (which may be of the static contact type) load resistance l2 and condenser 3. The rectified potential developed across resistance I2 is applied through a filter I 4, 5 to the control grid 16 of V4, and it will be seen that a decrease in amplitude of scanning currents induced in coil l0 will cause the grid potential of V4 to fall, thereby decreasing its anode current. It is clear that a control potential could be obtained from the line scanning valve V1 in a similar way, but according to a feature of the invention the potential developed across resistance It is used to supply operating current to the relay coil i and the control valve V4. The resistance It forms part of a resonant return circuit previously referred to, such circuit including also condenser I7 and diode valve V2 connected across the primary H! of the line scanning transformer TL. The function of this circuit has already been described in the Specification of British Patent No. 400,976 and it will be sufiicient here to state that the direct current potential developed across resistance l6 depends upon the amplitude of the line scanning oscillations. This potential is applied to the electrodes of V4 as indicated in the diagram, viz. the major portion is applied between anode and cathode while nega tive bias for the grid is obtained by returning it to a point on It below the cathode connnection.

Under normal working conditions the control valve V4 receives a net grid bias of approximately zero due to the resultant of the positive potential developed across resistance [2 and the negative potential obtained from part of resistance 56. The anode potential derived from resistance i6 via coil 4'and resistance I8 is then sufiicient to drive a current through V4 sufficient to keep contact 3 closed. If, however, the line scanning oscillations partly or wholly fail the potential across resistance l6 falls and becomes insuificient to drive enough current through the circuit of V4 to maintain the relay 3, 4, closed. The decrease of anode current and resulting rise of anode potential of V4 renders V5 conducting as previously explained to discharge the high tension line 9.

It is thus seen that a complete or partial failure of either 01' both of the scanning oscillations will result in the relay contact 3 opening to disconnect the supply mains from the apparatus and to discharge the high tension line 9 through the valve relay V5, thus preventing damage to the fluorescent screen of the cathode ray tube.

Referring to Figure 2 of the drawings a transmitting tube I of the iconoscope type is shown provided with deflection coils S1. and SF for line and frame scanning respectively. These coils are supplied with scanning currents from time bases shown diagrammatically at L and F. The iconoscope tube is supplied with high tension voltages from a power supply unit R. In this case the high voltages are arranged to be negative with respect to ground the (relatively) highest po tential accelerating electrode A2 being grounded while the cathode C and focussing electrode A1- from the high tension lines 2t and 2! and tov con nect the cathode C to earth throughout contact current from any convenient source of potential 13 and the anode current through it is regulated by automatic control of the grid bias by rectifier circuits 23 and 24 coupled to the line and frame scanning outputs as shown. When either or both the line and frame scanning oscillations decrease unduly the sum of the rectified potentials developed by circuits 23 and 24 decreases;

and the resulting fall of grid potential of Vireduces the current through the relay coil 4 thus causing contact 3 to open. Contact 3 is arranged 'to operate a more powerful relay 25 having contacts 26 which disconnect the high tension lines from the tube and further contacts 22 which discharge the cathode of the tube to earth.

It is important that the time constants of the resistance capacity circuits associated with the rectifiers 23 and 2l should be carefully selected vention, potentials derived from the line and frame scanning circuits are applied respectively to the two control grids of a valve of the hexode type which is adapted to control a suitable relay or relays serving to disconnect the H. T. supp-1y to the cathode ray tubes and preferably effective- 13? to discharge rapidly the high potential on the accelerator or first anode of the cathode ray having further screening or suppressor or other grids. The well known names hexode,. heptode, octode, are applied to such valves, and their essential characteristic for the purpose of the invention is that the two control grids in series in the electron stream are each separately capable of controlling the anode current. These types of valves will be referred to'as valves of the hexode type. The two control grids are generally each biased negatively with respect to the cathode,

and circuits embodying, hexode valves are described in the Specification of British Patent No. 423,213.

Preferably, the control grids of the hexode valves are initially biased substantially to anode current cut-01f, or beyond cut-01f, and the potentials derived from the scanning circuits are such as to cause an increase of anode current from the initial (or zero) value. The anode current or potential is caused to operate a relay or relays in any suitable manner( Preferably, a current-operated relay is connected in the anode circuit of the hexode and is arranged to break the H. T. connection to the cathode ray tubes when the anode current falls below a predeter mined value, and to short circuit theH. T. supply. It will be appreciated that the anode In this case V4 is supplied'with operating,

current of the hexode depends upon the potentials applied to both of the control grids and consequently will fall in the event of failure of either or both of the scanning generators. A voltage operated relay may also be controlled by the potential developed across a resistance included in the anode circuit of the hexode, and.

may be arranged to discharge an electrode of the C. R. tube in response to a fall of anode current of the hexode.

According to one feature of the present invention, the hexode valve is arranged as a rectifier so that the scanning oscillations can be applied directly to the control grids instead ofv being applied to auxiliary rectifier circuits serving to derive D. C. potentials to be applied to the said control grids. For example, the hexode may be adapted to operate as an anode bend rectifier.

The operating potential for the hexode may be obtained from the source which supplies the scanning valves. Failure of this source will thus cause the anode current of the heXode to fall or cease, with consequent operation of the relays. Further, the relay serving to discharge an elec trode of the cathode ray tube, which relay may be a thermionic valve, is supplied with operating potential from the source which supplies the said electrode of the cathode ray tube.

Referring to Figure 3 of the drawings, the hexode valve 3 has applied to its control grids scanning oscillations derived from resistances d and 5 in series respectively with scanning coils 6 and '1. The anode circuit of the hexode includes a relay R which controls contacts in the H. T. supply for the cathode ray tube, and a load resistance 8, the potential across which is applied to a valve relay 9 adapted to discharge the accelerator electrode of the cathode ray tube. The anode circuit of the hexode also includes a shunt condenser H! which presents a lowim-v pedance to currents of scanning frequencies. In the cathode lead are a resistance H and condenser I2, which provide grid bias for the valve by means of the passage through I l of the steady component of the valve current and of the current in the potentiometer -lil3 connected across a'source of H. T. Condensers l2 and M are by-pass condensers for the oscillatory components of the anode and screen grid currents. The potentiometer may be designed so that the valve 3 is initially biased to orbeyond anode current cut-off. The scanning oscillations applied to the control grids cause a flow of anode current which maintains the relay contacts closed and develops a potential across 3 which. renders the valve 9 non-conducting. An undue fall in amplitude of either or both of the scanning oscillators results in a fall of anode current of the valve 3 upon which the relay opens andthe valve 9 becomes conducting, thus discharging the accelerator electrode down to the anode supply voltage of valve 3. 1

v The'purpose of the condenser 10 in the anode circuit of the hexode valve is to smooth the and as the time constants in the line scanning output circuits are small, the spot on the oathode ray tube screen will be undefiected in a horizontal direction for most of this period and is likely to burn a vertical line on the screen. An improvement in this respect can be obtained by making the time constant of the anode circuit of the heXode of the order of a few lines and to allow one grid of the hexode to operate as an anode bend rectifier for line scanning oscillations, but to insert a separate rectifier and smoothing circuit between the frame scanning circuit and the other grid of the hexode, said smoothing circuit having a time constant of the order of two frame periods.

Referring now to Figure 4 of the drawings there is illustrated a modification of Figure 3 in which corresponding elements have been similarly numbered. The circuit dilfers from said Figure 3 in two respects:

Firstly, the scanning potentials applied to the grids are derived from small current transformers l6 and H associated with the scanning coils 6 and E. It is preferable to place the transformers (or the resistances in Figure 3) in the junction of the parts of the scanning coils as shown so that short-circuit of the coils will result in cessation of the potentials applied to the grids of valve 3, with consequent operation of the relays. It will be clear to one familiar with the art that other methods of applying scanning potentials to the valve 3 may be devised; for example, coils in the grid circuits of valve 3 may be coupled to the deflection coils on the cathode ray tube, or existing or extra windings on the scanning output transformers can be used.

Secondly, the resistance 8 which controls the thermionic relay valve 9 is placed in the cathode to earth return lead so that the cathode of valve 9 is nearly at earth potential. In this case the valve 9 is capable of discharging the accelerator electrode of the cathode ray tube almost down to earth potential. Further, the cathode-heater insulation of valve 9 will not be subjected to a large difference of potential. This modification is not the preferred arrangement where only a small change is produced in the cathode current by the outer control electrode. Similar precautions with regard to the time constants apply also to this circuit.

Referring to the modification shown in Figure 5 of the drawings the line frequency signals and the frame frequency signals fed from sources denoted by L and F respectively are fed to transformers and the outputs of said transformers are applied to the circuits embodying rectifiers 23 and 24 respectively. The rectified outputs from these circuits are applied through suitable resistances to the grid of a double-diode triode valve, the anode 32 of the triode portion of the double-diode triode having a relay coil 4 in its circuit so that a switch contact 3 is made during normal working, when sumcient current is flowing in the coil of relay 4 and the contact connects the H. T. supply to one or more of the electrodes of the cathode ray tube, not shown in this figure. It is preferable that the anode 32 of the valve be supplied with current from the same source that supplies the scanning generators so as to protect the tube if the source fails.

The purpose of the two diodes 38 and 3! in conjunction with resistances 28 and 29 is to limit the maximum output from the rectifiers 23 and 24 to the potential of the battery 21. Thus, should the output of the diode rectifier 24 increase so that the potential of diode 3! equals that of the battery 21 the diode conducts and thus limits the potential of the diode anode to that of the battery 21. Similarly, diode 30 limits the potential of the output of the rectifier 23. Because resistances 33 and 34 are equal in value the potential of the grid of the valve is the mean value of the potentials of the two diode anodes. If the bias 2'! is at least twice the value required to cut off the anode current of the valve it will be evident that failure of either of the scanning circuits will operate the relay 4, whatever the amplitude of the other scanning oscillations. In addition, the smoothing circuits associated with the rectifiers 23 and 24 may be chosen for quick operation as already outlined. A decrease in amplitude of either the line frequency oscillations or the frame frequency oscillations below a predetermined value, the amplitude of the other frequency oscillations remaining normal, will result in the double diode triode valve being rendered non-conducting with consequent failure of the high tension supply to one or more electrodes of the cathode ray tube. Additional means are provided similar to those already described whereby the high tension supply is short-circuited.

Although the invention has been described with reference to cathode-ray tubes with electromagnetic deflection, it may also be applied to cathode ray tubes with electrostatic deflection, suitable means being provided for applying the scanning potentials to the rectifiers or hexodes.

We claim:

1. Television apparatus comprising a cathode ray tube having means within said tube for generating a beam of electrons, said means including an accelerating anode, a rectifier circuit for supplying potential to the accelerating anode, means for deflecting the beam of electrons in two mutually perpendicular directions, means to supply the deflecting means with two potentials of predetermined amplitude, means to derive a potential from each of the two potentials, means actuated by the derived potentials to de-energize the rectifier circuit whenever one of the two po- 2. Television apparatus comprising a cathode ray tube having means within said tube for generating a beam of electrons, said means including an accelerating anode, a rectifier circuit for supplying potential to the accelerating anode,

means for deflecting the beam of electrons in two mutually perpendicular directions, means to supply the deflecting means with two potentials of predetermined amplitude, means to derive a potential from each of the two potentials, thermionic means actuated by the derived potentials to deenergize the rectifier circuit whenever one of the two potentials falls below a predetermined value and separate thermionic means actuated by the derived potentials to abruptly and directly discharge the rectifier circuit simultaneously with it being deenergized.

3. Television apparatus comprising a cathode ray tube having means within said tube for generating a beam of electrons, said means including an accelerating anode, a rectifier circuit for supplying potential to the accelerating anode, means for deflecting the beam of electrons in two mutually perpendicular directions, means to supply the deflecting means with two potentials derive a-potential from each of the two potentials, means actuated by the derived potentials to de-energize the rectifier circuit whenever one of the two potentials falls below a predetermined value and separate thermionic means actuated by the derived potentials to abruptly and directly discharge the rectifier circuit simultaneously with it, being deenergized.

4. Television apparatus comprising a cathode ray tube having means within said tube for generating a beam of electrons, said means including an accelerating anode, a rectifier circuit for supplying potential to the accelerating anode, means for deflecting the beam of electrons in two mutually perpendicular directions, means to supply the deflecting means withtwo potentials of predetermined amplitude, rectifier means to derive a potential from each of the two potentials, thermionic means actuated by the derived poten tials to de-energize the rectifier circuit whenever one of the two potentials falls below a predetermined value and separate thermionic means'actuated by the derived potentials to abruptly and directly discharge the rectifier circuit simultaneously with it being deenergized.

of predetermined amplitude, rectifier means to l derive a potential from each of the two potentials, a thermionic tube having a cathode and anode and at least two control electrodes, means to supply one of the control electrodes with one of the derived potentials, means tosupply the other control electrode with the other of the derived potentials, means to de-energize the rectifier circuit whenever the potentials supplied to the deflecting means fall below a predetermined value and separate thermionic means actuated by the derived potentials toabruptly and directly erating a beam of electrons, said means includ-- ing an accelerating anode, a rectifier circuit for supplying potential to the accelerating anode,

means for deflecting the beamof electrons in two mutually perpendicular directions, means to supply the defiecting means with two potentials of predeterminedamplitude, rectifier means to derive a potential from each of the two potentials, means for comparing the two derived potentials, thermionic means actuated by the difference between the two derived potentials for de-energizing the rectifier circuit and separate, thermionic means actuated by the difference between the two derived potentials for abruptly and directly discharging the rectifier circuit simultaneously with it being de-energized.

ERIC LAWRENCE CASLING WHITE. CHAR ES. LESLIE FAUDELL. 

